Subsurface safety valves generally have a flapper that is closed by a torsion spring that is mounted on a pivot pin for the flapper. A hydraulic control system actuates a piston to move a flow tube in the valve passage against the flapper to hold it open. If pressure in the hydraulic system is removed or lost, the closure spring acts on the flow tube to lift it away from the flapper that until that time had been behind the flow tube in a recess in the housing. Once the flow tube moves up the torsion spring in the flapper pivot shaft would do the work of starting rotational movement of the flapper toward its conforming seat. When the flapper contacted the seat the pressure of the fluid below kept the flapper in that closed position sealed against the flapper seat. Pressurizing the control system again brought the flow tube against the closed flapper and made it pivot off the seat back to the open position.
As safety valves were made with larger flow bores and dealt with higher velocities particularly in gas service transient vortexes were formed of high pressure zones that changed location depending on the velocity. At certain flow passage dimensions and flow velocities these high pressure zones occurred in front of an open flapper to create a sufficient hold open force that the torsion spring was unable to move the flapper to the closed position even after the flow tube was raised to allow such flapper movement.
In the past, in addressing the larger sized flapper safety valves and the limitations of the torsion spring to move an ever heavier flapper, designs were developed along the lines of providing an assist to the torsion spring to start the flapper moving toward the closed position when the flow tube was raised up. U.S. Pat. No. 6,227,299 used a leaf spring 122 located behind the flapper 86 to add a closing force. US Publication 2009/0151924 uses a shape memory alloy closure spring to get a boost in the flapper closing force. Going in the opposite direction, U.S. Pat. No. 7,703,532 holds the flapper open with movably mounted magnets and U.S. Pat. No. 7,270,191 provides a mechanism to open the flapper when it will not go from the closed to the open position with the hydraulic system. US Publication 2009/0032238 uses repelling magnets in the housing and the flapper to give an assist to a torsion spring on the flapper pivot pin. U.S. Pat. No. 7,448,219 is a hingeless flapper design that shapes the flapper to be aerodynamic so that it can operate responsive to the flow passing by in an automotive application. U.S. Pat. No. 7,644,732 uses a bypass technique for dealing with pressure surges in a lubrication system when the circulating oil is still cold.
The various solutions discussed above have in common a focus on adding a closing force when it is time for the flapper to go to the closed position. The present invention addresses the configuration of the flow passage to reduce or eliminate the effect of flow induced pressure transients that can overcome the ability of the flapper torsion spring to close it in high velocity fluid flow situations in the order of 300 feet per second or higher. Rather than adding to the mechanical closing force applied to the flapper, the present invention focuses on dissipation of flow induced moving pressure gradients that can act on the flapper at the time it needs to close and reducing their affects by shaping the profile of the flow passage in the vicinity of the flapper or the flapper itself so that the localized pressure differentials are not large enough to overcome the torsion spring trying to close the flapper. Those and other aspects of the present invention will become more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is provided by the appended claims.